I love using a thread color that shows up & pops on the quilt backing. It creates a reversible quilt. I quilt on a longarm with pantographs. It's a good thing others don't know what the design is supposed to look like. lol Ironically, everyone I show my quilts to, like the fronts but love the backs. I know before I give them the quilt that they are going to use it "upside down" because they like the simplicity & the starkness of the contrasts between the quilt back color & the thread.
There are NO quilt police. Do it like you want. Another irony, I match the quilting thread to the front so that it doesn't take away from the piecing & the quilt pattern. My material in my backs are always found on the front of the quilt too. This way there is no shock from a front that's pink & brown to a back that's yellow. Using a brown back with pink thread is stunning!